It is known to issue electronic vouchers which may be exchanged for goods and services. European Patent Application EP 0 823 694 (KPN) discloses electronic “tickets” or vouchers which may be stored in a smart card, while International Patent Application WO 00/30045 (KPN) discloses electronic vouchers which may also be printed. The vouchers of WO 00/30045 contain identification data for identifying the service to be rendered.
These electronic vouchers may be purchased using regular money and may then be stored on a smart card which a user carries in her purse. The smart card is used at a point of sale to submit the voucher in exchange for goods or services. Instead of electronic vouchers, electronic coins are sometimes used.
A problem that may occur with electronic vouchers or electronic money is fraudulent use: a user may be able to spend the same voucher or coin twice. The electronic vouchers of WO 00/30045 are “stamped” when used so as to prevent their being used more than once. However, fraudulent users may find ways to prevent or even undo this “stamping” and use the same voucher again. Accordingly, a fraudulent user may commit fraud repeatedly.
U.S. Pat. No. 4,987,593 (Chaum) discloses an electronic cash system using digital signatures. Certain information contained in the electronic cash can be recovered under certain conditions, thus allowing to identify fraudulent users of the system. However, fraud may only be detected when a user spends an electronic cash item more than once (“double spending”). This does not stop a fraudulent user from receiving new electronic cash, nor is there a guarantee that the user uses the goods or services obtained with the cash in an honest and legal manner. Accordingly, this known system is capable of identifying double spending users but may still issue new electronic cash in case of fraud.